Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Many transmissions are designed to operate at a discrete number of fixed speed ratios. The available speed ratios may be referred to as gears or gear ratios even if there is not a direct correspondence between each ratio and a particular physical gear within the transmission. The ratios are typically numbered starting from the highest speed ratio and progressing to lower speed ratios. For example, 1st gear may have a speed ratio of 4.5, 2nd gear a speed ratio of 3.0, 3rd gear a speed ratio of 2.3, etc.
A particular speed ratio from the set of available speed ratios may be selected by engaging particular shift elements such as clutches or brakes. Shift elements may include actively controlled devices and passively controlled devices like one way clutches. When the currently selected speed ratio is not optimal, the transmission shifts to a different speed ratio by releasing one or more shift elements and engaging one or more shift elements. For example, if the currently selected speed ratio results in an engine speed which higher than optimal, the transmission may upshift to a lower speed ratio. On the other hand, if the engine speed is excessively slow, the transmission may downshift to a higher speed ratio. During the shift, vehicle occupants may notice variations in output torque and may notice changes in the engine sound. Vehicle occupants may be annoyed if the shift is rough and jerky, if a shift occurs when they weren't expecting one, or if no shift occurs when they do expect one.
Modern automatic transmissions utilize controllers both to decide when to shift and to control the shift elements to accomplish the shift as smoothly as possible. Sometimes, a controller schedules a shift event in direct response to an action performed by the driver such as an abrupt change in accelerator pedal position. In these circumstances, the driver may expect that the transmission will shift and will be pleased to feel a shift. At other times, a controller schedules a shift event due to a gradual change in vehicle speed or pedal position. Occupants expect some shifting of this type but are displeased if the transmission shifts too frequently. When a vehicle is cruising over level terrain at a steady speed, vehicle occupants expect the transmission to remain in the currently selected gear. Occupants may believe that a transmission shift during such steady state driving is an indicator of something wrong with the transmission.